Biologia, Bratislava, 61/Suppl. 20: S401—S411, 2006 Section Zoology DOI: 10.2478/s11756-007-0076-6

Holocene subfossil chironomid stratigraphy (Diptera: ) in the sediment of Plešné Lake (the Bohemian Forest, Czech Republic): Palaeoenvironmental implications

Jolana Tátosová 1,JosefVeselý2 & Evžen Stuchlík3

1Institute for Environmental Studies, Faculty of Science, Charles University in Prague, Benátská 2,CZ-12801 Prague, Czech Republic; e-mail: [email protected] 2Czech Geological Survey, Geologická 6,CZ-15200 Prague 5, Czech Republic 3Hydrobiological Station, Institute for Environmental Studies, Charles University in Prague, P.O. Box 47,CZ-38801 Blatná, Czech Republic

Abstract: A faunal record of chironomid remains was analyzed in the upper 280 cm of a 543 cm long sediment core from Plešné jezero (Plešné Lake), the Bohemian Forest (Šumava, B¨ohmerwald), Czech Republic. The chronology of the sediment was established by means of 5 AMS-dated plant macroremains. The resolution of individual 3-cm sediment layers is ∼115 years and the analyzed upper 280 cm of the sediment core represent 10.4 cal. ka BP. As the results of DCA show, two marked changes were recorded in the otherwise relatively stable Holocene chironomid composition: (1) at the beginning of the Holocene (ca. 10.4–10.1 cal. ka BP) only oligotrophic and cold-adapted taxa (Diamesa sp.,M.insignilobus-, H. grimshawi-type) were present in the chironomid assemblages, clearly reflecting a cool climate oscillation during the Preboreal period, and (2) during an event dated in the interval 1540–1771 AD, when most taxa vanished entirely and only Zavrelimyia sp. and Procladius sp. were alternately present accompanied by Tanytarsus sp. Although, the age of this event is in agreement with the dating of the Little Ice Age, the most probable reason for the elimination of many chironomid taxa was very low sums recorded in this part of the sediment, rather than cool conditions connected with the LIA. Variations in the chironomid fauna after the Preboreal period were reflected mainly by changes in abundances of dominant taxa rather than by changes in composition. These variations could be explained by: (1) climatic changes, namely temperature and amount of rainfall resulting in oscillations in lake level, with changes in the occurrence of macrophytes in the littoral and (2) increasingly dense afforestation which led to a considerable input of organic material into the lake and a subsequent increase in the trophic status of the lake water. Key words: Chironomidae, glacial lake, climate changes, palaeoenvironmental reconstruction, palaeolimnology.

Introduction et al., 1997; Brooks & Birks, 2000). Chironomid lar- vae possess chitinized head capsules that are resistant Larvae of Chironomidae (Insecta: Diptera) colonize all to decomposition. Consequently, fossilized chironomid freshwater systems, from large lakes to the smallest head capsules tend to be well preserved in lake sed- ponds, and often dominate the benthos. This family iment over thousands of years, and can generally be is noted for its taxonomic richness, with nearly 10,000 identified to or, more rarely, to species-group species distributed globally (Cranston, 1995), repre- level. This offers the possibility of using the fossil chi- senting more than 20% of all freshwater in rivers ronomid record to infer past environmental conditions and lakes. As a result of their short generation times in lakes. Knowledge of modern chironomid ecology al- and the dispersal capacity of the winged adults, chi- lows the use of chironomid subfossils for the recon- ronomids respond rapidly to changes in a wide variety struction of palaeoproductivity (Brodersen & Linde- of environmental variables (Walker, 2001). Transfer gaard, 1999), acidification (Brodin & Gransberg, functions have been developed for a range of environ- 1993; Schnell & Willassen, 1996), palaeosalinity mental parameters including salinity (Heinrichs et al., (Walker et al., 1995) and, more recently, summer tem- 2001), dissolved oxygen (Quinlan & Smol, 2002) and perature variations (Brooks & Birks, 2001; Kor- nutrients (Brodersen & Lindegaard, 1999; Brooks hola et al., 2002; Heiri et al., 2003; Larocque & et al., 2001); however, over large climatic gradients, air Hall, 2004; Heiri & Lotter, 2005). temperature is often the best explanatory variable for The preliminary results presented here form part chironomid distribution (Lotter et al., 1997; Walker of a multi-disciplinary project investigating the biotic

c 2006 Institute of Zoology, Slovak Academy of Sciences S402 J. Tátosová et al.

Table 1. Radiocarbon dating of plant remains.

Laboratory code Depth (cm) CAR yr. BP Range 1) cal. ka BP Intercept with cal. ka BP 2)

NZA–9645 51–54 2005 ± 60 2.11–1.82 1.95 NZA–9317 105–108 3637 ± 60 4.13–3.82 3.95 NZA–13686 114–117 3949 ± 50 4.52–4.24 4.42 NZA–11663 141–144 4733 ± 55 5.59–5.32 5.53 NZA–9599 234–237 8264 ± 65 9.43–8.99 9.28

Explanations: CRA – Conventional Radiocarbon Age; NZA – Laboratory Code of the Rafter Radiocarbon Lab., New Zealand; 1) 95% confidence thresholds of ka BP; 2) REIMER et al. (2004).

and abiotic responses to climate changes that occurred but did not survive the period of maximum acidification and during the Late Glacial and Holocene in the Bohemian at present the lake is fishless. Forest (Šumava, B¨ohmerwald), Czech Republic. Similar studies are available for many areas of Europe (Gan- Methods douin & Franquet, 2002; Korhola et al., 2002; Porinchu & Cwynar, 2002; Lotter & Birks, 2003; In 1991, a 543 cm long core was collected by R. Schmidt (In- Brooks & Birks, 2004; Larocque & Hall, 2004; stitute for Limnology of the Austrian Academy of Sciences, Langdon et al., 2004; Dalton et al., 2005; Velle Mondsee, Austria) with a modified Kullenberg piston corer et al., 2005), but have never been done on lakes of close to the deepest part of the lake. The core was sub- glacial origins in the Czech Republic. Similar studies sampled every 3 cm and samples were continuously stored at 4 ◦C. A faunal record of chironomid remains was analyzed were published by Veselý et al. (1993), Veselý (1998, in the upper 280 cm. For analyses, a known weight of wet Pražáková & Fott Bitušík & ◦ 2000), (1994) and sediment was deflocculated in 10% KOH at 60 C for 20 min Kubovčík (2000). However, all of these concentrate and then washed with distilled water onto sieves with 230 on changes during shorter periods than are presented in µmand86µm mesh-size in order to facilitate the sorting this paper. The first integrated information on the mod- and picking of head capsules. The material was transferred ern chironomid fauna in these lakes of the Bohemian from the sieves into a plankton counting tray and all head Forest was given by Bitušík & Svitok (2006). capsules were picked with either fine forceps or a needle × The main aim of this paper is to describe the using a stereomicroscope at 25 magnification. After dehy- dration in isopropyl alcohol, head capsules were mounted on Holocene chironomid stratigraphy of Plešné jezero  slides in Euparal mounting medium. (Plešné Lake) and to provide a preliminary Holocene mainly follows WIEDERHOLM (1983), reconstruction of key environmental conditions in this SCHMID (1993), BITUŠÍK (2000) and RIERADEVALL & area. BROOKS (2001). Members of the tribe Tanytarsini were re- viewed using the description given by HEIRI et al. (2004). Study site Concentrations of organic carbon (C) in lyophilized sub-samples were analyzed with a TOC 5000A analyzer. Plešné Lake is a small lake of glacial origin (7.6 ha, max. depth 17 m) located at an altitude of 1087 m a.s.l. and Chronology situated in a relatively remote and uninhabited part of A total of seven terrestrial plant remains were isolated from the Bohemian Forest (48◦47 N, 13◦52 E) in C Europe. five levels of the Holocene sediment and dated by the AMS- Thebedrockofthecatchment(0.67km2) is formed by radiocarbon method in the Rafter Radiocarbon Laboratory, Lower Hutt, New Zealand in the year 2000. Errors of dating granite, and is covered with a thin layer of lithosol, pod- 14 zol and spododystric cambisol (JANSKÝ et al., 2005). The by C-methods are ≤ 200 years and the age of the core catchment is steep and forested, with Picea excelsa domi- bottom was ∼14.5 ka BP. The resolution of individual 3-cm nating. The average annual and July air temperatures are sediment layers is ∼115 years in the Holocene (Tab. 1; for 4.4 and 13.1 ◦C, respectively (Czech Hydrometeorological more details see PRAŽÁKOVÁ et al., 2006). Institute), and the period of ice-cover averages 135 days −1 −1 yr . The present average TP loading (11 µgL ) suggests Data analysis mesotrophic conditions in the lake. The chironomid percentage diagrams were created using Plešné Lake was partially affected by atmospheric acid- TILIA version 2.0.2 (GRIMM, 2004) and the zones were de- ification in the early 1960s, and during the peak of acidifi- fined using the CONISS (constrained incremental sum of cation in 1980s the pH of lake water dropped below 4.7 squares cluster-analysis) program available in TILIA. In ad- (VESELÝ &MAJER, 1996; KOPÁČEK et al., 1998). At the dition, Detrended Correspondence Analysis (DCA) was ap- present time, the lake is in the process of chemical and bi- plied to the chironomid data. DCA was performed using the ological recovery (MAJER et al., 2003; VRBA et al., 2003; program CANOCO version 4.5A (Ter BRAAK &SMILAUER, th NEDBALOVÁ et al., 2006). At the end of 19 century, Salmo 2003) and square-root transformation of species data and trutta L., 1758 was introduced to the lake (VESELÝ, 1994), down-weighting of rare taxa were used. Subfossil chironomids from Plešné Lake S403

Results mulative variance in the chironomid data. The sample scores of these axes were plotted on a time scale, with Taxonomy scaling in the original standard deviation units. The We distinguished 35 different taxa, most of which were first DCA axis (Fig. 1) separated faunal assemblages identified to genus level using mentum characteris- before ca. 10.2 ka BP from younger samples. On axis tics (Dicrotendipes, Microtendipes, Pagastiella, Pseu- 2, decreases in values of sample scores were recorded dochironomus, Cladotanytarsus, Parakiefferiella, Za- between 1.5 ka BP and the present, with a significant lutschia, and Diamesa). However, when mandibles or event in the period 1540–1774 AD. The Holocene sedi- premandibles were not available and the mentum was ment was divided into 6 chironomid zones in agreement damaged or worn down, larger taxonomic groups were with the results of CONISS cluster–analysis (Fig. 2). formed in the cases of Limnophyes/Paralimnophyes, Cricotopus/Orthocladius, and Chironomus/Einfeldia. ZONE-1 (276–267 cm, 10.4–10.1 cal. ka BP) For a more synoptic display in the graphs, these In this zone, taxonomic richness was low; nevertheless, groups include also specimens assigned to the genera. In the total sum of individuals per sample reached the some instances, taxa were identified to species-groups; maximum value of the entire sediment profile. The chi- Corynoneura scutellata-type was determined by the dis- ronomid community consisted mainly of cold-adapted tinct pattern on the head capsule surface, and a men- taxa. The dominant chironomid taxon was Procladius tum with 3 median teeth and well defined first lateral (∼50%) followed by Heterotrissocladius grimshawi-type teeth; Heterotrissocladius marcidus and H. grimshawi- (∼30%) and Micropsectra insignilobus-type (∼20%). type were separated on the basis of the specific submen- TheoccurrenceofDiamesa sp. was recorded only in tum color; Psectrocladius psilopterus-type was identi- this zone, though in very low densities. At the top of fied based on a mentum with two median teeth with the zone, the proportion of Corynoneura scutellata-type nipple-like projections that do not exceed the lateral increased and some other taxa such as Heterotrissocla- teeth in length, and Psectrocladius sordidellus-type by dius marcidus, Microtendipes sp. and Zavrelimyia sp. a mentum with two wide apically pointed median appeared sporadically. Members of Procladius recorded teeth exceeding the lateral teeth in length. Remains in this zone probably belong to a different species than of Micropsectra were determined as M. insignilobus- the specimens in the upper part of the sediment core. type when a short spur was present on the antennal pedestals, the postoccipital plate was well developed, ZONE-2 (267–198 cm, 10.1–7.8 cal. ka BP) and the mandible had three inner teeth, one apical and The cold-adapted taxon H. grimshawi-type from the one outer tooth. When available, the bifid premandibles previous zone persisted in lower densities at the be- were used to confirm the Micropsectra genus. Tanytar- ginning and later gradually disappeared. Procladius sp. sus lactescens-type included specimens with the combi- formed only ca. 10% of the chironomid community nation of mandibles with two inner teeth and a straight in Zone 2. Corynoneura scutellata-type became dom- blunt spur on the antennal pedestals. Head capsules of inant (∼40%) at the bottom of this zone. The sec- Procladius were identified mainly on the base of the ond most numerous taxon was H. marcidus (∼15%). cephalic setation, because the lingulae were often miss- Towards the top of the zone, the abundance of C. ing. Unfortunately, no remains of Procladius pupae, scutellata-type decreased to ca. 30% and H. marcidus which have better characteristics for species identifica- increased to ca. 25%. Other important components of tion (Langton, 1991), were found. Nevertheless, head the chironomid fauna in this zone were Microtendipes capsules of Procladius separated from the oldest lay- sp. and Zavrelimyia sp., which were present in the ers of the sediment were up to double in size compared whole zone at more or less constant abundances of to specimens found in the rest of the sediment core. ca. 15 and 10%, respectively. Also, other temperate or This could point to two different species of Procladius. warm-adapted taxa were found, with abundances less genus B had a very wide slightly split than 5% (Parakiefferiella sp., Dicrotendipes sp., Crico- median tooth on the mentum not exceeding the first topus/Orthocladius). The appearance of Psectrocladius lateral teeth and lighter than the four lateral teeth. We sordidellus-type was recorded only in this zone. The did not find any remains of Chaoboridae in the sedi- total number of individuals per sample remained rela- ment. tively high.

Chironomid stratigraphy ZONE-3 (198–156 cm, 7.8–6.1 cal. ka BP) The analyzed part (276 cm) of the sediment core from Cold–adapted taxa, such as M. insignilobus-type and Plešné Lake covers the period from 10.4 cal. ka PB to H. grimshawi-type, disappeared entirely. The chirono- the present. Chironomid data were analyzed using DCA mid fauna was still dominated by H. marcidus and in order to assess the compositional structure and taxa C. scutellata-type, with Microtendipes sp., Zavrelimyia turnover throughout the core profile. DCA calculated a sp. and Procladius sp. present at lower abundances gradient length of 2.8 standard deviation units and pro- (∼10%). The ratio of dominant taxa changed across this duced two significant axes that explained 30% of the cu- zone- abundances of C. scutellata-type gradually de- S404 J. Tátosová et al. Fig.1.

Fig. 1. A comparison of chironomid results with selected sedimentological parameters. The chironomid data are summarized in the first two axes of DCA. Axes scales are given in standard deviation units. The sum of head capsules is expressed as the number of individuals per gram of a wet weight of the sediment. creased as H. marcidus gradually increased. Some taxa such as Psectrocladius psilopterus-type, Cladotanytar- (Phaenopsectra sp. or Limnophyes/Paralimnophyes) sus sp. and Cricotopus/Orthocladius were present ir- occurred irregularly, and reached maximum abun- regularly and in very low abundances. Dicrotendipes dances of almost 10%. With relatively stable abun- sp. completely vanished in the second half of the dances lower than 5%, P. psilopterus-type, Cladotany- zone, whereas Chironomus/Einfeldia appeared in no- tarsus sp., Dicrotendipes sp., and Cricotopus/Orthocla- table numbers. The total sum of individuals per sam- dius were present. ZONE–3 is characterized by a ple remained in the same low range as at the top of the marked decline in the total sum of chironomids. previous zone.

ZONE-4 (156–84 cm, 6.1–3.1 cal. ka BP) ZONE-5 (84–42 cm, 3.1–1.5 cal. ka BP) Although Orthocladiinae dominated across the entire The ratio of the most abundant taxa again changed; sediment profile, an increased proportion of Chironom- C. scutellata-type varied between 10 and 30%, whereas inae was found in this zone as a result of higher H. marcidus gradually decreased down to less than 5% abundances of Microtendipes sp. and Phaenopsectra sp. at the zone top. Zavrelimyia sp. and Procladius sp., Head capsules of H. marcidus and C. scutellata-type also abundant species, persisted in relative stable abun- still formed the main part of the chironomid assem- dances across this zone, as did Microtendipes sp. and blages; nevertheless, a significant decline was recorded Phaenopsectra sp. However, these taxa were present in for C. scutellata-type at the top of this zone, as well lower numbers than in the previous zone. A peak of as a high variability in the abundance of H. mar- ca. 20% was recorded for Limnophyes/Paralimnophyes cidus. Also, abundances of Zavrelimyia sp., Procladius in the middle of the zone, but its occurrence was ir- sp. and Parakiefferiella sp. varied across this zone and regular elsewhere, as was the presence of Parakief- these taxa almost disappeared at the zone top, whereas feriella sp., Cladotanytarsus sp. and Dicrotendipes sp. distinct increases of Limnophyes/Paralimnophyes and In the second half of the zone, Ablabesmyia sp., Tany- Ablabesmyia appeared at the same time. Other taxa, tarsus lactescens-type, Psectrocladius psilopterus-type Subfossil chironomids from Plešné Lake S405

er of the peak in their id assemblage composition. ronomids per sample and are arranged in ord

diagram. The zones define major changes in the chironom ected taxa are displayed as percentages of the total chi Fig. 2. Chironomid stratigraphy of Plešné Lake. Sel abundances from bottom-left to top-right across the S406 J. Tátosová et al. and Cricotopus/Orthocladius appeared. The total sum genus Chironomus, which is adapted to the low oxy- of individuals per sample remained in the same low gen concentrations, with other taxa generally prefer- range as in the previous zone. ring colder, well oxygenated conditions such as Zavre- limyia melanura/barbatipes, Heterotrissocladius mar- ZONE-6 (42–0 cm, 1.5 cal. ka BP to ∼1950 AD) cidus, Psectrocladius sordidellus (Bitušík & Svitok, This zone is characterized by an event that happened 2006). The recent chironomid fauna consists mainly of between ca. 18 and 10 cm (1540–1771 AD), when most members of the Orthocladiinae, and this group domi- of the taxa totally disappeared. Only three taxa per- nated across the whole Holocene sediment core as well sisted, with highly variable abundances. Zavrelimyia sp. (Fig. 2). This suggests that the trophic status of the dominated (more than 40%) at the beginning of this lake could have been oligotrophic to mesotrophic, but event and then was replaced by Procladius sp. Tany- probably never became eutrophic in its history, as is tarsus lactescens-type reached two peaks here, the first also evident from the analysis of cladoceran remains less distinct and coinciding with an increase in Zavre- (Pražáková et al., 2006). The changes in the chi- limyia sp. and the second more pronounced and associ- ronomid assemblages throughout the core took place ated with higher abundances of Procladius sp. mainly through changes in abundance of the following Before this event, at the 10–18 cm depth, the chi- taxa: Corynoneura scutellata-type, H. marcidus, Micro- ronomids were represented by two types of assemblages. tendipes sp., Procladius sp. and Zavrelimyia sp. Larvae At the beginning of Zone 6 the fauna consisted mainly of Procladius live in lakes with a range of trophic and of Microtendipes sp., Limnophyes/Paralimnophyes, Za- temperature conditions (Wiederholm, 1983; Lotter vrelimyia sp. and Procladius sp., whereas C. scutellata- et al., 1998); nevertheless, the very high abundance of type, Phaenopsectra sp. and Parakiefferiella sp. were this genus at the beginning of the Holocene (ZONE- less abundant. This assemblage was accompanied by a 1) (Fig. 2) suggests that these specimens were cold group of low abundant (∼5%) taxa such as Cladotany- stenothermous species of the Procladius genus. The tarsus sp., Dicrotendipes sp., Cricotopus/Orthocladius, genus Diamesa are typically rheophilous taxa and their Pagastiella sp. and Zalutschia sp. (first occurrence). occurrence in lake sediments is usually linked to an Members of Tanytarsus sp. were missing. Immediately increased amount of precipitation in the lake catch- preceding the event at 10–18 cm depth, the domi- ment, when the bodies of these taxa could be trans- nant taxa were C. scutellata-type, Phaenopsectra sp., ported into the lake by tributary waters (Ruck¨ et Parakiefferiella sp. , Limnophyes/Paralimnophyes,and al., 1998). Cricotopus/Orthocladius, C. scutellata-type Tanytarsus sp. The members of the accompanying and Dicrotendipes are littoral taxa usually associated group vanished. with aquatic macrophytes, which can provide evidence After the 10–18 cm depth event, C. scutellata- of lake level fluctuation (Brooks, 2000). Taxa such type, Microtendipes sp., Zavrelimyia sp., Procladius sp., as Diamesa, Heterotrissocladius grimshawi and Mi- Parakiefferiella sp. and Chironomus sp. were all present cropsectra insignilobus-type are typical cold adapted at ca. 10%; the other taxa such as Phaenopsectra sp., taxa (Säwedal, 1982; Sæther, 1975, 1979). In Limnophyes/Paralimnophyes, Dicrotendipes sp., Crico- addition, H. marcidus or Zavrelimyia spp. can indi- topus/Orthocladius and Tanytarsus sp. followed with cate colder well-oxygenated conditions (Fittkau, 1962; abundances of 5%. In addition, H. marcidus again ap- Sæther, 1975; Lotter et al., 1997) as the anal- peared in this community. yses of the recent chironomid fauna from the Bo- hemia Forest lakes has confirmed (Bitušík & Svitok, Discussion 2006). Ablabesmyia, Parakiefferiella, Microtendipes and Chironomus are considered to be temperate or warm Plešné Lake is unique among lakes of the Bohemian adapted taxa, and particularly Microtendipes and Chi- Forest. As analyses of the recent chironomid fauna ronomus indicate an increased trophic status of lake show (Bitušík & Svitok, 2006), lakes located at waters. On the other hand, H. grimshawi, Micropsectra lower altitude are inhabited by a more diverse chi- insignilobus, Pagastiella and H. marcidus reflect gen- ronomid fauna than lakes at higher elevations (Laka erally oligotrophic conditions in lakes (Wiederholm, Lake, Prášilské Lake). Although Plešné Lake is among 1983; Brundin, 1949). Some taxa of the genus Psec- the higher situated lakes, its chironomid species rich- trocladius become abundant in acid lakes (Brodin, & ness is relatively high. One reason is likely the dif- Gransberg, 1993), so their presence can be connected ferent geology of the lake catchment. Plešné Lake lies to changes in lake-water pH. Since P. sordidellus-type on granitic bedrock, while other lakes are situated on presently inhabits Plešné Lake, in which the pH of sur- mica schist. According to Kopáček et al. (2006), the face lake water can fall below 5 during the brief pe- granitic bedrock results in a different soil composition, riod of the spring snow/ice melt, this species can be and consequently can lead to higher terrestrial phos- considered to be an acid-tolerant taxon. On the other phorus input into the lake. Mesotrophic conditions in hand, this taxon was also found to be abundant in many the lake are probably the reason for the higher chi- lakes in the Alps with pH of about 8 (Lotter et al., ronomid diversity as well as the co-existence of the 1998). Subfossil chironomids from Plešné Lake S407

An environmental interpretation of major changes in ginning of this zone there are two cooling oscillations, the chironomid stratigraphy as is reflected in the decline in organic carbon con- In order to assess compositional structure and taxa centrations (Fig. 1). These oscillations also correspond turnover throughout the profile, we applied DCA to the well with the persisting cold-adapted H. grimshawi.As chironomid data. DCA produced two significant axes was mentioned above, H. marcidus and Zavrelimyia are from which the sample scores were plotted on a time able to tolerate colder conditions in lakes of the Bo- scale, with scaling in the original standard deviation hemian Forest (Bitušík & Svitok, 2006), so their rel- units (Fig. 1). As the results of DCA show, the chi- atively high abundances may not preclude a cool cli- ronomid composition of Plešné Lake was relative stable mate, though not as cold as is supposed during the during the Holocene; nevertheless, two notable changes previous period, in which these taxa were missing. In were found. Firstly, in the period between ca. 10.4 and addition, a distinct increase in the abundances of C. 10.1 cal. ka BP, the highest scores on the first axis oc- scutellata-type and a relatively high proportion of Mi- curred. Next during the interval from 1540–1771 AD, crotendipes sp. in the chironomid community indicate the sample scores of both axes significantly decreased. warmer conditions during this period. As is evident At the /Holocene transition, four cli- from the finding of Iso¨etes in the Boreal sediment of mate cooling oscillations were documented in the sedi- Plešné Lake (Jankovská, 2006), the development of ment of Plešné Lake (11.2, ∼10.5, 10.2–10.1 and 9.6 cal. macrophytes in the littoral part of the lake probably ka BP, respectively), and were clearly identified by de- took place at this time. This corresponds well with the creases of organic carbon concentrations (Pražáková presence of Corynoneura scutellata-type, Dicrotendipes et al., 2006). ZONE-1 (10.4–10.1 cal. ka BP) represents and Cricotopus/Orthocladius in this period. Increasing a ca. 300 yr-long period between the 2nd and 3rd Prebo- humic acid inputs during this period are suggested by real cooling oscillations. As is known from analyses of the brief appearance of Psectrocladius sordidellus-type the recent chironomid fauna, both the species compo- in this zone. sition and low diversity recorded in this zone resemble Subsequent significant decreases in abundances of the chironomid fauna in cold and nutrient poor high- C. scutellata-type and Microtendipes, and a marked in- mountain and sub-arctic lakes (Bretschko, 1974; crease of H. marcidus in the period between 7.8–6.1 Aagaard, 1986; Cameron et al., 1997; Rierade- cal. ka BP may indicate increasing lake water level vall & Prat, 1999; Tátosová & Stuchlík, 2006). (Brooks, 2000) and input of allochthonous material The presence of entirely cold adapted taxa, such as Di- (Warwick, 1989) because of very humid environment amesa sp., M. insignilobus-type, H. grimshawi,andthe during the Late Atlantic (8–6 ka BP, Svobodová et generally very low species diversity in ZONE–1 reflect al., 2002). The following period of the middle Holocene the cool climate conditions of this period. The high is characterized by an overall decrease in abundances rate of sedimentation (Fig. 1) recorded at the start of of the two previously dominant taxa C. scutellata-type this zone was probably the result of an input of al- and H. marcidus and an increased proportion of Mi- lochthonous material from the lake catchment having crotendipes sp. and Phaenopsectra sp. in the chirono- open vegetation. A subsequent drop of the sedimen- mid community. Furthermore, all present taxa showed tation rate by ∼50% (Fig. 1) was most likely caused high fluctuations in their abundances. The structure by the afforestation of the originally treeless watershed of the chironomid assemblages reflect well the unsta- by birch and pine forest (Jankovská, 2006), when the ble conditions of the Epiatlantic epoch (6–4.5 ka BP, roots of trees slowed physical erosion in the steep catch- Svobodová et al., 2002), in which wet and dry pe- ment of Plešné Lake. An increased supply of organic riods alternated and the climate was generally very material, caused by the expansion of vegetation around warm. The subsequent disappearance of C. scutellata- the lake, could have allowed the appearance of Chirono- type, Dicrotendipes and Cricotopus/Orthocladius at mus at the border of ZONE-1 and ZONE-2. Although the end of this period corresponds with a consider- this genus is considered to prefer warm conditions, lar- able decline in macrophyte pollen found in this part vae of Chironomus anthracinus-type are often present of sediment. As the pollen analyses also showed, the in lakes after deglaciation (Brooks & Birks, 2000; population of the quillwort Iso¨etes, the only macro- Heiri et al., 2003; Larocque & Hall, 2004). This phyte documented during the Holocene, were not abun- taxon is probably better adapted than Chironomus dant throughout the Holocene history of Plešné Lake plumosus-type to tolerate cooler conditions (Brooks, (Jankovská, 2006). At present, it occurs only in the 2000). shallow part of the lake near the inlets, down to depth The following ZONE-2 (10.1–7.8 cal. ka BP) cov- of 0.5–1 m. Therefore, it is possible to suppose that ers the period from the Preboreal / Boreal transition a decline of the lake level by more than 1–2 m dur- to the Holocene climatic optimum. The Boreal (9–8 ing the very dry episode at the end of the Subboreal ka BP, Svobodová et al., 2002) is characterized by (4.5–3 ka BP, Svobodová et al., 2002) could have forest development (Corylus and mixed oak woods) led to the elimination of macrophytes as well as as- (Jankovská, 2006) which caused a continuing low sed- sociated chironomids. This could also have been the iment accumulation rate over this period. At the be- cause of the higher abundances of the semi-terrestrial S408 J. Tátosová et al. species Limnophyes/Paralimnophyes (Moog, 1995) in been documented in pollen analyses by Jankovská, the lake sediment. The increased proportion of Chirono- (2006). The very high sedimentation rate in the layers mini tribes in this period corresponds well to warm and 10 cm and up (ca 18th century) indicates high physical higher trophic lake water conditions. erosion resulting from deforestation of the lake catch- The period between 3.1–1.5 cal. ka BP is followed ment. by a gradual replacement of H. marcidus with Pro- Plešné Lake, as well as many lakes in C and N Eu- cladius sp. in the profundal of the lake. One possible rope, was affected by atmospheric acidification at the explanation could be that the persisting higher input 1970s and 1980s of the 20th century (Fott et al., 1987, of nutrients and declining lake depth (even if only by 1994; Kopáček et al., 1998). Although the resolution about 2 m) resulting from sedimentation, could have of the sediment core is relatively low for detailed analy- led to anoxic or low oxygen conditions in the profun- sis of the effects of acidification, a distinct change in the dal part of Plešné Lake. These conditions would prob- chironomid community was recorded in the upper layer ably have been unfavorable for larvae of H. marcidus, of 3–0 cm (1956–1990 AD). Most of the taxa present a taxon living in oligotrophic lakes. The continuous oc- in the previous layer disappeared and the chironomid currence of pollen of Plantago lanceolata and Rumex fauna has generally remained poor. Again, the prob- acetosella, indicating grazing, shows that the develop- lem of a low chironomid concentration and accompa- ment of cultural fields and settlements may have in- nying lower species diversity in this part of sediment creased the trophic status of the lake water during this records could explain this trend, but nevertheless acid- period (Jankovská, 2006). Also, the presence of other ification is likely to be at least partially responsible. warm adapted, mesotrophic-preferring taxa, such as Di- The strong effects of acidification on the biological com- crotendipes, Cladotanytarsus or Ablabesmyia could con- munity are well apparent from analyses of cladoceran firm the mesotrophic status of Plešné Lake during this remains (Pražáková et al., 2006). period. On the other hand, the reason for the decreased abundances of Microtendipes sp. or Phaenopsectra sp. Conclusions compared to the previous zone is uncertain. The structure of the chironomid assemblage in the The chironomid composition throughout the core was uppermost part of the sediment core showed distinct relatively stable during the existence of Plešné Lake, changes. The most marked changes took place in the mainly reflecting crucial changes in the Holocene cli- period between 1540 and 1771 AD (18–10 cm) when mate. As shown by the DCA analyses, the species com- most taxa entirely vanished and only Zavrelimyia sp. position of early Holocene sediment samples was dis- and Procladius sp. were alternately present, accompa- tinctly different from younger samples; the presence nied by Tanytarsus sp. This event is in agreement with of only oligotrophic and cold-adapted taxa in this as- the dating of the Little Ice Age, and could be the re- semblage clearly reflects a cool climate oscillation of sult of low oxygen concentrations at the lake bottom the Preboreal period. The variations in the chirono- caused by longer winter ice cover (Lindegaard, 1995; mid fauna after the Preboreal period were mainly rep- Heiri & Lotter, 2003). However, these sediment lay- resented by changes in abundances of dominant taxa ers also coincide with an episode of very low chironomid rather than by changes in species composition. Oscilla- abundances (Figs 1, 2), which makes the fossil chirono- tions in the lake level and changes in the occurrence mid record less reliable. Moreover, samples with a low of macrophytes in the littoral, both connected with number of specimens usually tend to have lower species climate changes, namely temperature and amount of diversity. It is likely, therefore, that this decrease in chi- rainfall, provide possible explanations for variances in ronomid abundances is at least partly related to the low the chironomid fauna. Another reason for chironomid count sums rather than to cool summer temperatures. changes was most likely the development of vegetation On the other hand, the structure of chironomid as- in the lake catchment, resulting in a distinct decline semblages recorded before the above-mentioned period in erosion and sedimentation at the Preboreal/Boreal was much more varied (layers 33–27 cm, 938–1212 AD). transition. Later, the increasingly dense afforestation The well developed littoral community with the dom- led to a considerable input of organic material into the inant taxon Microtendipes sp. accompanied by Dicro- lake and a subsequent increase in the trophic status of tendipes sp., Corynoneura scutellata-type and Cricoto- the lake water, which is evidenced by a significantly pus/Orthocladius associated with macrophytes, infers a higher proportion of Chironomini in the chironomid warmer climate such as has been described in the Mid- fauna over the period of the Middle Holocene. These dle Ages. Also, the occurrence of Chironomus/Einfeldia mesotrophic conditions have persisted to the present, in the profundal zone of Plešné Lake confirms the as is evident from the upper sediment layers where warmer conditions in the lake and probably low oxygen Heterotrissocladius marcidus was replaced by Chirono- concentrations at the bottom. The most likely reason mus/Einfeldia in the profundal community. This grad- for this increase in lake water trophy was the heavy hu- ual progression of the chironomid fauna was interrupted man impact on the forest structure, and agriculture and by an event in the period 1540–1771 AD when the ma- landscape exploitation of the lake surroundings, as has jority of taxa entirely disappeared. This could be re- Subfossil chironomids from Plešné Lake S409 lated to the cool conditions connected with Little Ice bodenfaunistischen Charakterz¨uge schwedischer oligotropher Age dated in this period, though it is also possibly due Seen. Rep. Inst. Freshwater Res., Drottningholm 30: 1–914. to the very low count sums recorded in this part of the CAMERON,N.,FJELLHEIM,A.,RIERADEVALL,M.,RADDUM, G.G., SCHNELL,O.,FOTT,J.,STUCHLÍK,E.,ČERNÝ,M. sediment. &KOPÁČEK, J. 1997. Contemporary biology, pp. 1–60. 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